Ball screws include a screw shaft, with a helical screw groove on its outer circumference, and a nut, formed with a helical screw groove on its inner circumference, to engage the screw shaft. A large number of balls are accommodated within a rolling track formed by the opposite screw grooves. A circulation mechanism enables the rolling track to be a circulating track. The ball screws are utilized as motion conversion mechanisms to linearly drive the screw shaft by rotating the nut.
In general, there are various types of ball circulating mechanisms used in the ball screws. One of which is a bridge-type. In the bridge-type ball screw, the bridge member for circulation of balls, is mounted on the nut. It has a linking track to enable the rolling tracks to be a circulation track. Its structure is relatively simple and compact.
One representative example of a bridge-type ball screw is shown in FIGS. 7(a) and (b). A barrel of a nut 53, of the bridge-type ball screw, is formed with a circular bridge window 56. The window 56 passes through the outer and inner circumference surfaces of the nut 53 while cutting out part of screw grooves 53a. A circular bridge member 55 is fit into the bridge window 56. The bridge member 55 is formed by a MIM (Metal Injection Molding) method. A linking groove 55a, for connecting mutually adjacent screw grooves 53a, is formed on the inner surface of the bridge member 55. This forms a rolling track for balls 54 consisting of the linking groove 55a and the screw groove 53a of substantially one lap. A large number of balls 54 are accommodated within a space between the screw groove 53a of the nut and a screw groove 52a of a screw shaft 52. The balls 54, rolling along the screw groove 53a, are guided to the linking groove 55a of the bridge member 55. The balls 54 ride over a screw thread of the screw shaft 52 and return to the screw groove 53a and continue to roll therealong again.
The linking groove 55a of the bridge member 55 has an S-shape. This smoothly connects two mutually adjacent screw grooves 53a of the nut 53. Accordingly, the linking groove 55a is joined to the screw grooves 53a so that edges 57 of the linking grooves 55a, of both edges of the bridge window 56, meet groove edges 58 of the screw groove 53a, as shown in FIG. 7(b). In addition, arms 59, each having a round rod-like shape, are projected from both sides of the bridge member 55. The arms 59 engage the nut screw grooves 53a via a predetermined radial gap. The arms 59 are able to axially position the bridge member 55 relative to the nut 53. The arms 59 prevent the bridge member 55 from radially slipping out from the bridge window 56.
In addition, the bridge member 55 has a recessed radially outer surface 60 and a pair of guide walls 61. The guide walls project radially outward from the recessed surface 60 at circumferentially opposite positions of the bridge member 55. The bridge member 55 is secured to the nut 53 by a caulked part 62. The caulked portion 62 is formed by plastically deforming the guide walls 61 using a caulking tool (not shown). This reduces the manufacturing cost while reducing the machining steps. Also, it provides a bridge-type ball screw that can exactly and easily set the axial position of the bridge member 55 relative to the nut 53. Thus, this improves its reliability (see JP2007-146915 A).
In such a bridge-type ball screw, processing time of the bridge window 56 can be reduced by making the shape of the bridge window 56 circular. However, the circular bridge window 56 has a problem in that it increases surface pressure of the screw groove 53a near the bridge member 55 since a portion of the screw groove 53a on which the balls 54 roll is loaded based on the contact angle when an axial load is applied to the screw shaft 52, as shown by an arrow in FIGS. 8(a) and (b). That is, the contact ellipse caused by rolling of balls 54 on the screw groove 53a is usually allowed by the screw groove 53a. However, since an edge part (part having an acute angle) of the bridge window 56 is present in the nut screw groove 53a leading into the bridge member linking groove 55a (see FIG. 7), it is believed that an excessive load, so-called “edge load”, is created on the edge part of the bridge window 56 in the screw groove 53a by action of the contact ellipse onto the edge part when the balls 54 roll to the bridge member 55, as shown by an enlarged drawing of FIG. 8(c).
The inventor of the present disclosure has noticed the joining part, between the screw groove and the ball circulation part, such as the linking groove of the bridge member, forms a non-load area. It is an object of the present disclosure to provide a long life ball screw that reduces the edge load caused on the screw groove near the ball circulation part and thus prevent premature flaking of the screw groove due to excessive surface pressure.